Typical semiconductor devices include regions of material having n-type conductivity, in which electrical current is carried principally by electrons, and material having p-type conductivity, in which electrical current is carried principally by electron vacancies, commonly referred to as xe2x80x9choles.xe2x80x9d The semiconductor device is connected to an external electical circuit by contacts. For example, a light-emitting diode includes p-type and n-type regions and a junction between these regions. A contact referred to as the p-contact is provided on the p-type region, whereas a contact referred to as the n-contact is provided on the n-type region. When an electrical voltage in the proper direction is applied between these contacts by an external power source, a current flows between the contact. Electrons in the n-type region and holes in the p-type region move toward the junction and combine with one another at or adjacent to the junction to produce light.
In most semiconductor devices, the contacts should exhibit xe2x80x9cohmicxe2x80x9d characteristics. That is, the electrical voltage loss at the boundary between the contact and the semiconductor material should be substantially proportional to the current, and should be independent of the direction of current flow, so that the contact acts as a conventional electrical resistor. Also, the contact desirably has low-resistance. For example, a light emitting diode with low-resistance ohmic contacts can convert electrical power into light more efficiently than a similar diode with high-resistance contacts. The contacts typically are connected to metallic leads as, for example, by wire-bonding processes. The contacts should include metals which are compatable with these processes.
The materials which provide low-resistance ohmic contacts vary with the composition and conductivity type of the semiconductor in the device. Certain semiconductor devices are formed from III-V materials, i.e., compounds of formed from a material in Group III of the periodic table and a material in Group V of the periodic table as, for example, gallium nitride (GaN) and similar materials. Contacts for p-type GaN commonly include gold or combinations of gold and nickel. For example, contacts for use on light-emitting diodes commonly include an electrode formed from thin layers of gold and nickel abutting the p-type GaN and a thick pad region including a layer of gold at the top surface of the pad. When the electrode is annealed it becomes transparent so that light emitted within the LED can pass out of the device through the electrode. The gold-containing pad provides a surface suitable for wire bonding using gold wires. The pad covers only a small portion of the device surface.
Lin et al., Low Resistence Ohmic Contacts On Wide Band Gap GaN, 64 (8) Applied Physics Letters February 1994, at 1003-1005, and U.S. Pat. No. 5,563,422 disclose contacts for n-type GaN formed from titanium, aluminum, or both, which is annealed. The ""422 patent states that if titanium and aluminum are provided in a multi-layer structure, deposition of titanium should be performed first, followed by deposition of aluminum. A contact including only titanium and aluminum would be incompatable with gold wire bonding. Accordingly, the ""422 patent suggests covering the Ti/Au structure with a xe2x80x9chigh-melting point metallic materialxe2x80x9d preferably including gold as a topmost layer, and preferably including another high-melting metal such as titanium and/or nickel.
Despite these and other efforts in the art, still further improvements would be desirable. For example, where a gold layer is provided on a contact containing titanium and aluminum, the gold layer can change during annealing. These changes impair the reliability of the wire bonds. It would be desirable to provide a contact and contact-forming method for n-type GaN and other n-type III-V semiconductors which would provide a low-resistance ohmic contact and which would also allow reliable bonding of gold leads such as gold wires to the contact.
One aspect of the invention provides a method of forming a contact on an n-type III-V semiconductor comprising the steps of depositing a base layer formed predominantly from Al on an n-type III-V semiconductor; then depositing a first barrier layer formed predominantly of one or more first barrier metals selected from the group consisting of Ti,Ta and Pd on the base layer; then depositing a second barrier layer formed predominantly of one or more second barrier metals selected from the group consisting of Pt, W and alloys of Ti and W on said first barrier layer. The method according to this aspect of the invention further includes the step of depositing a top layer formed predominantly of one or more top metals selected from the group consisting of Au and Ag on said second barrier layer, whereby said layers form a stack on the n-type semiconductor.
The deposited layers form a stack on the n-type semiconductor. The n-type III-V semiconductor with the stack is annealed to form the contact.
The III-V semiconductor preferably is a xe2x80x9cnitride semiconductor,xe2x80x9d i.e. a III-V semiconductor in which N constitutes 50% or more, and preferably 80% or more of the group V element. The semiconductor more preferably is a gallium nitride based semiconductor, i.e., a nitride semiconductor including gallium as, for example, GaN, InGaN, AlGaN or AlInGaN. The annealing step typically is performed at a temperature of about 400-600xc2x0 C. for about 1-10 minutes. Most preferably, the base layer consists essentially of Al, the first barrier layer consists essentially of Ti, the second barrier layer consists essentially of Pt and the top layer consists essentially of Au.
Although the present invention is not limited by any theory of operation, it is believed that the barrier layers such as Ti and Pt layers above the Al-containing base layer prevent undesirable reactions between Al and the metal of the top layer Au during annealing and/or during service. Although the present invention also is not limited by any theory of operation in this respect as well, it is believed that the Al in the base layer abutting the n-type semiconductor diffuses into the semiconductor and/or forms intermediate materials at the boundary with the n-type semiconductor. Whatever the mechanism of operation, the resulting contact has a low resistance and ohmic behavior. There may be some alloying of Ti and Al, and/or some alloying of Pt and Au.
A further aspect of the present invention provides a semiconductor unit including n-type III-V semiconductor with an n-type contact made as discussed above. The unit may further include a lead, preferably a gold-containing lead such as a gold wire or a gold-covered lead formed from another material, bonded to the contact.